Abstract
In the Kapitza–Dirac effect, atoms, molecules, or swift electrons are diffracted off a standing wave grating of the light intensity created by two counter-propagating laser fields. In ultrafast electron optics, such a coherent beam splitter offers interesting perspectives for ultrafast beam shaping. Here, we study, both analytically and numerically, the effect of the inclination angle between two laser fields on the diffraction of pulsed, low-energy electron beams. For sufficiently high light intensities, we observe a rich variety of complex diffraction patterns. These do not only reflect interferences between electrons scattered off intensity gratings that are formed by different vector components of the laser field. They may also result, for certain light intensities and electron velocities, from interferences between these ponderomotive scattering and direct light absorption and stimulated emission processes, usually forbidden for far-field light. Our findings may open up perspectives for the coherent manipulation and control of ultrafast electron beams by free-space light.
Highlights
Coherent control of the shape of quantum wave functions has set the way towards bond-selective chemistry [1], quantum computing [2, 3], and ultrafast control of plasmons [4]
Quantum coherent control originates from the ability to manipulate the interference between quantum paths towards realizing a desired shape of a target wave function by means of shaped, coherent, and/or strong laser excitation
We show that the KD effect can be generalized to the realization of arbitrary momentum states of the electron wavepacket by controlling the interference between quantum pathways originating from distinctly different parts, absorptive and ponderomotive, of the interaction Hamiltonian
Summary
Coherent control of the shape of quantum wave functions has set the way towards bond-selective chemistry [1], quantum computing [2, 3], and ultrafast control of plasmons [4]. As a step in this direction, we show here how to use the elastic interaction of freeelectron waves with focused, freely-propagating laser-fields to coherently control the transversal distribution of electron wave packets This is achieved by a generalization of the Kapitza-Dirac (KD) effect [14, 15] to the concomitant utilization of standing-wave and travelling-wave light patterns. We show that the KD effect can be generalized to the realization of arbitrary momentum states of the electron wavepacket by controlling the interference between quantum pathways originating from distinctly different parts, absorptive and ponderomotive, of the interaction Hamiltonian This offers fundamentally new degrees of freedom for designing light-controlled phase masks for free-space electron pulses
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